De zoetwatermollusken van het duingebied ten noorden van Den Haag

This article, the result of investigations in the years 1966-1970, gives a survey of the molluscs which have populated the dune lakes and pools in the catchment area of the waterworks of the city of The Hague. These lakes and pools were formed by infiltration of Rhine water as from 1955. Before that time the area was dry with the exception of three localities. The hydrology of the infiltrated water is described, as well as general aspects of flora and fauna of the newly formed waters. Two types of water are distinguished here: type a — lakes which receive river water by means of a pipe-line, and type b — pools caused by rising of the ground water table. Drought periods caused by interruption of the infiltration greatly influence the local distribution of the molluscs. The shallow pools may dry out easily and completely. Deep, permanent pools harbour comparatively more species than shallow ones (table 3 and fig. 4). The molluscs will probably have populated the new lakes and pools in the dunes by means of transport by birds, among which duck are common, and aquatic insects (Rees, 1965). All species found are common in the Netherlands, except for Gyraulus laevis (see Kuijper, 1971). It is expected that in the future more species will migrate to this by now well watered dune area.